Inverter for generating single or multiphase current

ABSTRACT

An inverter for generating single or multiphase current, comprising series networks equal to the number of phases, such networks being connected to a DC source or sources. Each said series network includes two capacitors in series connection with the common junction between the two capacitors as one load terminal which is connectable to its respective phase load. At least one electronic commutator is provided for each series network, said commutator being connected to a respective second load terminal and being switched alternately between the ends of terminals of the series network. Means are provided for triggering the commutator in sequence with and at the proper phase of the frequency of the alternating current to be generated. The inverter provides a power source for inductive current consuming appliances, particularly induction motors.

United States Patent Guggi 51 Jan. 25, 1972 t [54] INVERTER FORGENERATING SINGLE 3,483,462 12/1969 Bedford.. ..321/45 0 MULTIPHASECURRENT 3,496,092 2/l970 Fraser ..32l/45 [72] Inventor: Walter B. Guggi,Niederglatt, Switzerland Primary Examinerwilliam P- t W lh l [73]Assignee: Regus AG, Regensdorf, Switzerland A mmey oodhams B am and Fynn [22] Filed: Apr. 14, 1969 B T A T [21] A L N 815,598 An inverter forgenerating single or multiphase current. comprising series networksequal to the number of phases, such networks being connected to a DCsource or sources. Each [30] Apphcatlon Prmmy Data said series networkincludes two capacitors in series connec- Apr. 19, 1968 Switzerland..5852/68 with the i between two capaciwrs as one load terminal which isconnectable to its respective phase [52] US. Cl ..32l/43 At least oneelectronic commutator is Provided for each [5 1] Int. Cl. ..H02m 7/52Series network Said commutator being connected m 3 respec' 5s 1 Field ofSearch ..321/2 43 44 45 load ermine! and being Switched alternatelybetween the ends of terminals of the series network. Means [56]References Cited are provided for triggering the commutator in sequencewith and at the proper phase of the frequency of the alternating UNITEDSTATES PATENTS current to be generated. The inverter provides a powersource 3 047 790 7/l962 Grosbau et al for inductive current consumingappliances, particularly iny m d t 3,120,633 2/l964 Genuit ....321/45 moms 3,303,406 2/1967 Bedford "321/45 x 19 Claims, 8 Drawing Figures3,406,330 10/1968 Pelly ..32l/45 X PATENTED JAN 2 51912 SHEET 1 [IF 5IFJ VENTUR.

M1475? 5. 6066/ )WflhM/Fg AVOKNEXS ab cd FIG.2

mvamon MM/JM AFdfA/EK? PATENTED JAN25-1872 3,638,088

saw u or 5 FIG.3

INVENTOR.

INVERTER FOR GENERATING SINGLE OR I MULTIPHASE CURRENT The inventionrefers to an inverter for generating single, or multiphase current,comprising series networks equal to the number of phases, and beingconnected to a DC source, each such series network including twocapacitors in series connection with the center tap that is, the commonjunction between the two capacitors, as one load terminal, beingconnected to its respective phase load, including at least oneelectronic commutator for each series network, being connected to itsrespective second load terminal, and being switched alternately betweenone of both terminals of the series network, as well as to contain meansfor triggering said commutator in sequence with and at the proper phaseof the frequency of the AC current to be generated and particularly toprovide a power source for inductive current consuming appliances,particularly induction motors.

Invertersof this type do not have electrical characteristics making themsuitable for direct connection to motor drive units. This excludes theapplication of such inverters for a large percentage of suchapplications. The limitations are due mainly to the additionalproduction as well as operating costs of such circuitry and matchingelements as would be necessary in combination with the invertersmentioned before, whenever AC motors and similar equipment are suppliedby such units. This would result in theaddition of low-pass filters, orbandpass filters, as well as current or voltage regulators for theapplication of such inverters in supplying power to AC motors andsimilar motor drive units, resulting in lower efficiency of the inverterdue to relatively highreactive currents circulating within the system.

An additional disadvantage results thereof by an increase in weightmaking such combinations relatively unsuitable for applications inportable equipment, such as for instance handtools.

The main reason for requiring such additional filter and controlcircuitry is the fact that'known inverters generate a current andvoltage waveform considerably departing from the ideal sine wave, aswell as being load dependent.

To obtain a sine-wavelike current and voltage output it becomesnecessary to add some form of circuitry, such as filters, tuned circuitswithin the inverter system. This again necessitated the addition ofstabilizing means within the inverter because such tuned circuitsnaturally cause instability with respect to load variations,necessitating compensating means to-eliminate destructive effects uponthe inverter elements, or its load by such excessive current or voltagevariations.

The purpose of the invention therefore was to build an inverter in whichsuch shortcomings as being present within known inverters, can beeliminated at the same time comprising relatively simple circuitrytogether with improved electrical characteristics, and at the same timereducing the numbers of necessary components, making this inverterparticularly suitable in combination with portable tools and. equipment.

According to this invention this is achieved for inverters previouslydescribed by means of constructive unification of an inductive load withcapacitors (2, 3) of the series network, respectively each seriesnetwork, and being of such electrical value that the capacitiveimpedance of each series network which is formed by the parallelconnection of each capacitor being part of such series networkcompensates at least partially the load impedance connected to thecenter tap of this series network, at half the trigger frequencygenerated by the electronic switching system.

A particularly advantageous connection for generating cons'ists insupplying galvanically separated phase windings e.g., induction motorsseparately, respectively by separating intertied connections offinishing motors with an inverter comprising the same number of seriesnetworks according to the number of phases, such series networks beingconnected to a common DC source, each circuit with two capacitors atwhich center tap a connection exists for one of each phase terminalscomprising a commutating switch connected to the other respective phaseterminal, so that any interconnection between phases is accomplishedthrough the inverter exclusively. This makes it possible to connect eachseries network belonging to the various phases in parallel to the DCsupply source. it is also possible to connect each series networkbelongingvto its respective phase in parallel to the DC source wheneverthe load includes galvanically separated windings interconnected bycapacitors, or as can be the case for twophase induction motors thephases may be already interconnected within the load.

If however the load contains a multiphase system of which the phases areinterconnected the advantages of this inverter can be utilized withinthe same system simply by providing separate DC power sources for eachphase of the inverter system. i

The advantages of this system can be used as well in a system comprisinga load with multiphase polygon, for example, delta connections wherebycurrent is supplied between 7 each tie point from series networks of aninverter system respectively its commutator connection which is part ofan inverter having a separate DC source supplying each inverter section.

For the purpose of generating a multiphase AC current supplying aninductive load of which the phase voltages are in star, for example,Y-connection having a common tie point the inverter system can beextended in such a way that separate DC sources are connected to eachseries network comprising two capacitors at which center connection thefirst load terminal is connected, such load terminal being preferablythe common connection of the Y-centerpoint and which inverter comprisesa number of commutating switches connected to the second load terminal,and being equal to the number of phases for which the switchingarrangements providecommutation between center connection and itsrespective phase, including provisions that interconnections betweenseries networks are provided exclusively through the inverter system.

To prevent voltage overshoots at the capacitors forming the seriesnetwork, which can easily happen under low load conditions, the invertercan be further improved by connecting diodes between the center tap ofthe series arrangement and the DC supply lines polarized in such a wayas to be nonconductive with respect to the DC voltage. Such diodes areadvantageously fitted in addition with series resistors.

The electronic commutating means applied to inverters of this typeadvantageously comprise two electronic switches which are alternatelydriven into conduction, one of which is connected between the supplybuss and one terminal of the series network belonging to suchcommutating means, while the second one is connected between the supplybuss and the second terminal of the series network. Such electronicswitches being advantageously of the thyristor types comprising meansfor triggering of each electronic switch at its predetermined timeinterval whereby thyristor pairs are being triggered alternately at halfcycle intervals. Furthermore it is of advantage to add a center tappedinductor in series at the center connections of each thy istor pair toprovide positive commutation therefore avoiding short circuiting of theDC supply source. Such a center tapped inductor generates an oppositevoltage of the conducting switching element upon the start of conductionof the nonconducting switching element and therefore turning off theconducting switching element. This arrangement can be further improvedby adding capacitance betweenthe center tapped inductor and the opposingconnections of the inverter switches. This addition improves the circuitin such a way as to increase the turnoff pulse necessary to bring thecommutating switch into nonconduction, providing faster and morereliable turnoff action.

nected in such a way as to be oppositely polarized with respect to theDC supply voltage.

It is known that an induction motor, or each phase thereof, can berepresented by an equivalent network incorporating an inductance andaresistance, wherein the inductance may be substantially constant whilethe resistance varies, as with the mechanical load on the motor. Aparticularly advantageous application of this inverter includes acombination with an AC motor particularly an induction motor. In thiscase the capacities of the series network respectively of eachindividual series network should be dimensioned in such a way that theinternal impedance of each network is equal to the inductive componentof the load impedance at half the switching frequency.

The DC power source may contain rectifiers fed from an AC source. Anumber of separate DC sources (for multiphase inverters) can be suppliedadvantageously over a transformer with separate windings being suppliedfrom a common AC source.

Inverters of this type supplied from DC sources comprising rectified ACsupplies can be used advantageously as frequency inverters andparticularly to transform AC current withline frequency into AC currentof a higher frequency. When supplying induction motors by such frequencyinverters it is possible to obtain much higher rotating speeds ascompared to mo tors connected to the line frequency such as for instance50 Hz. which provides a maximum motor speed of 3,000 rpm.

.Furthermore it is possible to provide continuous speed regulation ofinduction motors by changing the switching sequence of the electroniccommutator within the inverter. Normally such speed regulation is notpractical by other means for induction motors.

Based on the following figures the invention is described by twoexamples.

a FIG. 1 shows an inverter of the type described for generating asingle-phase AC current;

FIGS. 1A through 1D disclose examples of inverters of the type describedfor generating multiphase current.

FIG. 2 shows another example of an inverter of the type described forgenerating a two-phase current;

' FIG. 3 shows a cross section through an induction motor which isconstructively combined with an inverter of the type described; I FIG. 4shows a .schematic of a trigger device for generating triggering pulsesfor the thyristors, as indicated by the triggering device 6in FIG. 2. Iv v FIG. 1' shows the principleof the circuit of an inverter accordingto the invention, as can be used for generating a sin-- gle-phase ACcurrent. This. basic circuit is part of an electric motor or invertersystem, in which the inductor 1 serves as an electromechanicaltransducer, and at the same time serves as a means for generatingsine-wavelike current variations in connection with capacitors 2 and 3.To generate and maintain such current variations it may be of advantageto separate the capacitances into two separate units. Capacitors 2 and 3are in series connection between the positive'and negative pole of theDC power source 7. The center tap of this series connection ofcapacitors; that is, the common junction of the two capacitors, isconnected with one terminal belonging to the inductor while the otherterminal of this inductor is connected to the center tap of two seriesconnected electronic commutating switches which lie between the positiveand the negative poles of the DC source, and which are shown here asthyristors v 4 and 5. A triggering circuit 6 which can be of knownconfiguration furnishes triggering pulses to the gate electrodes oftator unit C.

In other words the phase angle present in the tuned circuit within theinverter as compared to the phase angle of the trigger pulses applied tothyristors 4 and 5 must be advanced to guarantee continuous operation ofthe system. The circuit shown in FIG. 4 has the main purpose todemonstrate the principle since it is not suitable to meet all therequirements of a complex motor drive system.

FIG. 2 shows an improved version of the half bridge inverter includingthe necessary modifications and improvements to make it suitable foroperation with an electric motor under various operating conditions. Thefollowing description serves the purpose of explaining such operatingconditions in connection with the new methods which serve to match thecircuit to the special performance requirements. The circuit shown inFIG. 2 contains the same basic elements as the circuit in FIG. I andincludes inductors l, capacitors 2 and 3, a DC source 7, a triggerdevice 6 and thyristors 4 and 5. Upon close examination of this improvedcircuit it is apparent that additional components have been added tomodify the operating performance of this circuit and to improve itsbehavior. These additional components include rectifiers 8, 9, 10 andI1, center tapped inductor l2, resistors 13, 14, 15, 16 and capacitor17. g

It must be pointed out, that this circuit combination can be used foreach desirable phase of a multiphase motor or any other multiphase motordrive system as well as any other multiphase load. In addition apractical circuit is shown for the DC source 7 to indicate a possibilityof using the inverter as a frequency converter. The DCcurrent isretrieved from an AC source such as a normal AC line. The DC source maybe composed of a bridge rectifier of circuit 7 together with a smoothingcapacitor 19, a switch 20 and a connecting line with a suitable plug 21.

Furthermore ordinary rectifiers can be replaced by SCRs in such quantityas required in order to adjust the output characteristic of the DCsource to the individual requirements of the inverter. Such SCRs canserve the purpose of automatic motor control, starting control, currentlimiting and other spe cial operating requirements. It is evident to theexpert that this special combination serves as a demonstrating exampleonly and may vary considerably in its practical configuration.

It is furthermore evident to the expert who is familiar with electricaland electronic drawing symbols in what special arrangements andconnections such elements can be used. The

following explanation shall serve the purpose of explaining by means ofa practical example, shown in FIG 2 the meaning of these additionalelements. FIG. 2 shows a particular circuit arrangement including asecond inverter system of similar design'in parallel with the firstsystem. This providesa twophase inverter system. Each of the parallelinverter circuits is capable to operate as'a single-phase inverterproviding the presence of appropriate triggering pulses. This combinedinverter system generates a two-phase current within the motor load.Within each inverter system inductor 1 represents a field coil of themotor. The only difference concerning the operation of this circuit ascompared to the circuit described above in form of a single-phase systemconsists in the time sequence of the trigger pulses generated bytriggering circuit 6 and the different distribution of such pulses tothe electronic switching elements. The two inverter systems receivealternate triggering pulses as required. FIG. 2 shows two differentpossible pulse sequences and their appropriate distribution andvariations thereof as applied to output a, b, c and d of the triggercircuit 6. Sequences a-c-b-d are possible as well as sequences ad-bc.Changing this sequence causes a change in rotational direction of themotorQThe pulse frequency generated within the trigger circuit is fourtimes the required output frequency of the inverter which feedsalternating current to the motor field coils 1. Trigger pulses arespaced at phase angle intervals of In case of a three-phase invertersystem the trigger pulse frequency would be six times the inverterfrequency whereby the trigger pulses would have a 60 spacing withrespect to the inverter frequency. Other multiphase inverter systemswould require suitable extended triggering arrangements.

The short description above serves the purpose of explaining the basicoperating principles of this circuit. A more detailed operatingdescription follows.

The essential elements of this improved inverter are shown in FIG. 2 andhave already been described shortly before.

As already described above an inverter circuit as shown in FIG. 1 isbasically suitable to operate within limited requirements. In thisparticular case however the inverter load consists of a motor fieldcoil 1. This circumstance alone makes it necessary to add additionalcircuit elements and members to assure proper performance under alloperating conditions.

The motor field coil which represents inductor 1 changes its impedanceconsiderably as a function of motor load. The field coils represent onlya part of the oscillating system within the inverter, therefore it isnecessary to apply certain compensating means to equalize its change ofelectrical characteristics under all operating conditions. Such meansare shown in FIG. 2, they consist of members 8 to 18. The function ofthese members is described during an entire cycle of current flow.

Assuming that thyristors 4 and 5 are nonconductive to start with andthat thyristor 4 receives the first trigger pulse to render itconductive, current starts to flow from the positive pole of the DCpower source 7 through thyristor 4 and the upper portion of inductor 12,through the motor winding 1. This current charges capacitor 3 while atthe same time capacitor 2 is being discharged followed by beingrecharged in opposite polarity. As is known from basic electronics sucha current assumes a sine wave function, provided that the circuitcontains nearly ideal inductive and capacitive reactances. Furthermoreit is known that the voltage across such capacitors can reach valueswhich are higher than the DC supply voltage before current flow ceases.Limiting diodes l0 and 11 with appropriate resistors 15 and 16 serve thepurpose to limit such voltages. The exact purpose of this networkhowever is described later.

As soon as the capacitors have reached their full charge and the currenthas reached zero value the current tends to change its direction andcapacitors 2 and 3 tend to discharge over the same current path, whichhas been used during the charging cycle. During this time periodthyristor 5 should be made conductive so that the charge of thecapacitors may flow back to the negative pole of the DC power source.Such thyristors can conduct current in one direction only therefore noreturn current can fiow back to the positive pole of the DC supplysource. For this reason a diode-resistor network 8, 9, 13 and 14 hasbeen added. Otherwise considerable overvoltages could build up acrossthe thyristors, which could result in their destruction. Sincethyristors have thyratronlike characteristics it is important thatcurrent flow stops within the circuit before the second thyristorbecomes conductive, so that short circuits across the two thyristorswhich are connected in series can be avoided. For this reason a centertapped choke 12 has been added having the purpose to generate anopposite voltage across the conducting thyristor by taking stored energyfrom capacitor 17 as soon as the nonconductive thyristor is madeconductive. In case current flow should not be completed when thenonconductive thyristor receives its trigger pulse a short circuit wouldbe generated resulting in an inverter fault.

It has been explained before that the time period of a complete currentcycle can be adjusted by proper selection of capacitors and inductors.For this particular application however the impedance of the inductor aswell as its phase angle are dependent on motor load and therefore canvary very considerably. For this reason means must be provided includingsuch items as inductor l2 and capacitor 17 to provide proper operationof the thyristors and proper commutation between thyristors 4 and 5under all operating conditions. It is even possible that at certainmotor speeds, particularly during the starting cycle as well as underheavy load conditions voltages can be generated within the motor winding1 which might add up to the voltages generated by the commutation cycleand which might increase or decrease the effective value of the totalvoltage. Such effects might cause starting difficulties for the motorpreventing it from reaching full speed and therefore making anoncompensated half bridge inverter unsuitable for such applications(see FIG. I).

A considerable improvement is achieved by adding the rectifier-resistornetwork 10, I], l5, 16. The basic influence of this network upon thecircuit consists of the elimination of induced voltages and theirdetrimental influences upon the operating characteristics of the motor.

A practical example shown in FIG. 2 shows such an inverter. Additionalmodifications are possible in order to adapt this circuit to thepractical requirements of various motor designs. These modifications mayinclude rectifier-resistance network 10, 1], l5, 16 which is connectedto the tap of inductor 1 respectively the motor winding, furthermore thecomplete elimination of the resistors in this rectifier-resistancenetwork or as well the parallel connection of capacitors to suchresistors. Whichever combination of network connections or variationsthereof is being chosen, depends upon the motor and the loadcharacteristics of the inverter-motor system. Its proper choice has anessential influence upon the optimum performance which can be achievedfor best operating characteristics of the apparatus. Considering thetwophase system in FIG. 2 it is apparent that an additional capacitor 18can be added between the two phases of the inverter system. Thiscapacitor may be in place of part of capacitors 2 and 3. This circuitmay bring certain advantages with respect to size, capacitor valuerespectively the physical dimensions of such capacitors with respect tothe total required capacitance.

Iclaim:

1. An inverter structurally combined with an inductive,current-consuming appliance for supplying alternating current thereto,comprising series network means including the same number of seriesnetworks as there are phases in the appliance, each series network beingconnected to a DC source and allocated to one phase, each series networkincluding two capacitors, a first connection for the particular phaseassociated with each series network at the common junction of said twocapacitors of said series network, a second connection for said phaseassociated with said series network, an electronic switching device foreach series network for joining said second connection alternatively toeach of both ends of said series network; and means for operating theelectronic switching device in step with the frequency of thealternating current to be produced and with a phase positioncorresponding to the phase switched, the capacitors of the individualseries networks being made such that the internal impedance of theseries network, formed by the two capacitors in the series network, andthe inductive component of the load impedance connected to the seriesnetwork, formed substantially by the part of the consuming applianceconnected to the common junction of the two capacitors of the seriesnetwork, offset each other at least in part at half the switchingfrequency of the electronic switching device or devices.

2. An inverter as in claim 1, for producing multiphase alternatingcurrent in which the phase voltages are not interlinked, for inductivecurrent consuming applicances having galvanically separated phasewindings, wherein all the series networks are connected in parallel to acommon DC source, each network being connected to a respective phasewinding through said first and second connections, said connectionsassociated with the individual phases not being interlinked, but beinginterconnected only through the inverter circuitry.

3. An inverter as in claim 1, for producing multiphase alternatingcurrent in which the phase voltages are interlinked by connection of thephase windings of the appliance to form a polygon and each phase windingterminates in and lies between two phase wires forming adjacent apicesof the polygon, wherein each series network is connected to its ownseparate DC source joined to the DC sources associated with the otherseries networks only through the inverter circuitry, each network beingconnected to its respective phase winding by a connecting pairconsisting of said first and second connections, said connecting pairassociated with the individual nating current in which the phasevoltages are interlinked by connection of the phase windings of theappliance to form a star, each phase voltage appearing across a phasewinding connected between a common neutral conductor constituting thenode of the star and a respective phase wire, wherein each seriesnetwork is connected to its own separate DC source joined to the DCsources associated with the other series networks only through theinverter circuitry, each network being connected to its respective phasewinding by a connecting pair consisting of said first and secondconnections, saidconnecting pairs associated with the individual phasesbeing interlinked to form a star, one wire in each said individualconnecting pair being connected to one of the phase wires and the otherwire in each said individual connecting pair being connected to theneutral conductor.

5. An inverter as in claim 1 for producing multiphase alternatingcurrent in which the phase voltages are at least partially interlinkedcapacitively for inductive current consuming appliances havinggalvanically separated phase windings, wherein all the series networksare joined in parallel to a common DC source, said first and secondconnections associated with the individual phases or phase voltagesbeing at least partially interlinked by capacitors.

6. An inverter as in claim 1, including a diode in series with an ohmicresistor inserted between said common junction and each of the two endsof each series network, the diodes being so polarized that they arebiased towards cutoff by the DC voltage across the ends of the seriesnetwork.

7. An inverter as in claim 1 wherein each of the electronic switchingdevices comprises two alternately operated electronic switches, one ofwhich is placed in the lead between the wire joining the currentconsuming appliance to the switching device and that end of the seriesnetwork associated with the latter, while the other is inserted in thelead between the wire joining the current consumer to the switchingdevice and the other end of the series network associated with thelatter.

8. An inverter as in claim 7, wherein the electronic switches consist ofthyristors, the means for tripping the electronic switch or switchessupply each thyristor with trigger pulses at intervals equal to theduration of one cycle of the AC it is desired to produce, and thetrigger pulses supplied to the two thyristors belonging to one and thesame switching device are separated by the duration of one half-cycle ofthe AC to be produced.

9. An inverter as in claim 7 including a center tapped impedance coilinserted between the two electronic switches belonging to the sameswitching device, the center tap of said impedance coil being joined tosaid second connection.

10. An inverter as in claim 7 including a capacitor connected betweensaid common junction of the series network and the consumer connectionof the switching device associated with the series network.

11. An inverter as in claim 7 including a diode in series with an ohmicresistor inserted between the consumer connection of the individualelectronic switches and each of the two ends of the series networkassociated with the switching device, the polarity of the diodes beingsuch that they are biased towards cutoff by the DC voltage across theends of the series network.

12. An inverter as in claim 1 wherein the load impedance or impedancescan be represented in each instance by an equivalent networkincorporating a substantially constant inductance and a variableresistance constituting the effective load or part thereof, the minimumvalue of which is less than the reactance of the inductance at half theswitching frequency of the switching device or devices, and wherein theinternal impedance of each single series network at half the switchingfrequency of the switching device is less than half the minimum yalue ofthe said yariable resistance.

13. An inverter as in claim 1 wherein the inductive current consumingappliance structurally combined with the inverter is an AC motor and inparticular an induction motor.

14. An inverter as in claim 13 wherein the capacities of the seriesnetwork is such that the internal impedance of each in-' dividual seriesnetwork at half the switching frequency of the switching device ordevices, when the AC motor is on full load, is equal to the inductivecomponent of the load resistance connected to the series network.

15. An inverter as in claim 1 wherein the DC source or each of the DCsources consists of a rectifier fed from an AC source.

16. An inverter as in claim 15 including several separate DC sources,each connected through a transformer having a separate secondary windingfor each DC source,'insulated from the other transformer windings, to acommon AC source.

17. An inverter as in claim 1 arranged as a frequency changer.

18. An inverter as in claim 17, in which the inductive current consumingappliance consists of an induction motor.

19. An inverter as in claim 18, in which, to regulate the running speedof the induction motor, the switching frequency of the electronicswitching device is varied.

1. An inverter structurally combined with an inductive, currentconsumingappliance for supplying alternating current thereto, comprising seriesnetwork means including the same number of series networks as there arephases in the appliance, each series network being connected to a DCsource and allocated to one phase, each series network including twocapacitors, a first connection for the particular phase associated witheach series network at the common junction of said two capacitors ofsaid series network, a second connection for said phase associated withsaid series network, an electronic switching device for each seriesnetwork for joining said second connection alternatively to each of bothends of said series network; and means for operating the electronicswitching device in step with the frequency of the alternating currentto be produced and with a phase position corresponding to the phaseswitched, the capacitors of the individual series networks being madesuch that the internal impedance of the series network, formed by thetwo capacitors in the series network, and the inductive component of theload impedance connected to the series network, formed substantially bythe part of the consuming appliance connected to the common junction ofthe two capacitors of the series network, offset each other at least inpart at half the switching frequency of the electronic switching deviceor devices.
 2. An inverter as in claim 1, for producing multiphasealternating current in which the phase voltages are not interlinked, forinductive current consuming applicances having galvanically separatedphase windings, wherein all the series networks are connected inparallel to a common DC source, each network being connected to arespective phase winding through said first and second connections, saidconnections associated with the individual phases not being interlinked,but being interconnected only through the inverter circuitry.
 3. Aninverter as in claim 1, for producing multiphase alternating current inwhich the phase voltages are interlinked by connection of the phasewindings of the appliance to form a polygon and each phase windingterminates in and lies between two phase wires forming adjacent apicesof the polygon, wherein each series network is connected to its ownseparate DC source joined to the DC sources associated with the otherseries networks only through the inverter circuitry, each network beingconnected to its respective phase winding by a connecting pairconsisting of said first and second connections, said connecting pairassociated with the individual phases being interlinked to form apolygon, both the wires in each said connecting pair being joined to twophase wires forming adjacent apices of the polygon and two wires, eachfrom two different ones of said connecting pairs, being joined to eachindividual phase wire.
 4. An inverter as in claim 1, for producingmultiphase alternating current in which the phase voltages areinterlinked by connection of the phase windings of the appliance to forma star, each phase voltage appearing across a phase winding connectedbetween a common neutral conductor constituting the node of the star anda respective phase wire, wherein each series network is connected to itsown separate DC source joined to the DC sources associated with theother series networks only through the inverter circuitry, each networkbeing connected to its respective phase winding by a connecting pairconsisting of said first And second connections, said connecting pairsassociated with the individual phases being interlinked to form a star,one wire in each said individual connecting pair being connected to oneof the phase wires and the other wire in each said individual connectingpair being connected to the neutral conductor.
 5. An inverter as inclaim 1 for producing multiphase alternating current in which the phasevoltages are at least partially interlinked capacitively for inductivecurrent consuming appliances having galvanically separated phasewindings, wherein all the series networks are joined in parallel to acommon DC source, said first and second connections associated with theindividual phases or phase voltages being at least partially interlinkedby capacitors.
 6. An inverter as in claim 1, including a diode in serieswith an ohmic resistor inserted between said common junction and each ofthe two ends of each series network, the diodes being so polarized thatthey are biased towards cutoff by the DC voltage across the ends of theseries network.
 7. An inverter as in claim 1 wherein each of theelectronic switching devices comprises two alternately operatedelectronic switches, one of which is placed in the lead between the wirejoining the current consuming appliance to the switching device and thatend of the series network associated with the latter, while the other isinserted in the lead between the wire joining the current consumer tothe switching device and the other end of the series network associatedwith the latter.
 8. An inverter as in claim 7, wherein the electronicswitches consist of thyristors, the means for tripping the electronicswitch or switches supply each thyristor with trigger pulses atintervals equal to the duration of one cycle of the AC it is desired toproduce, and the trigger pulses supplied to the two thyristors belongingto one and the same switching device are separated by the duration ofone half-cycle of the AC to be produced.
 9. An inverter as in claim 7including a center tapped impedance coil inserted between the twoelectronic switches belonging to the same switching device, the centertap of said impedance coil being joined to said second connection. 10.An inverter as in claim 7 including a capacitor connected between saidcommon junction of the series network and the consumer connection of theswitching device associated with the series network.
 11. An inverter asin claim 7 including a diode in series with an ohmic resistor insertedbetween the consumer connection of the individual electronic switchesand each of the two ends of the series network associated with theswitching device, the polarity of the diodes being such that they arebiased towards cut-off by the DC voltage across the ends of the seriesnetwork.
 12. An inverter as in claim 1 wherein the load impedance orimpedances can be represented in each instance by an equivalent networkincorporating a substantially constant inductance and a variableresistance constituting the effective load or part thereof, the minimumvalue of which is less than the reactance of the inductance at half theswitching frequency of the switching device or devices, and wherein theinternal impedance of each single series network at half the switchingfrequency of the switching device is less than half the minimum value ofthe said variable resistance.
 13. An inverter as in claim 1 wherein theinductive current consuming appliance structurally combined with theinverter is an AC motor and in particular an induction motor.
 14. Aninverter as in claim 13 wherein the capacities of the series network issuch that the internal impedance of each individual series network athalf the switching frequency of the switching device or devices, whenthe AC motor is on full load, is equal to the inductive component of theload resistance connected to the series network.
 15. An inverter as inclaim 1 wherein the DC source or each of the DC sourceS consists of arectifier fed from an AC source.
 16. An inverter as in claim 15including several separate DC sources, each connected through atransformer having a separate secondary winding for each DC source,insulated from the other transformer windings, to a common AC source.17. An inverter as in claim 1 arranged as a frequency changer.
 18. Aninverter as in claim 17, in which the inductive current consumingappliance consists of an induction motor.
 19. An inverter as in claim18, in which, to regulate the running speed of the induction motor, theswitching frequency of the electronic switching device is varied.